The narrow guage railway know as the "Tren a
las Nubes" snakes its way up the Quebrada del Toro toward the Puna. The
railway was remarkably well-constructed and has survived numerous floods,
landslides, and rockfalls. In addition to tourism the railroad still hauls
freight to San Antonio de los Cobres, the principal town on the Puna salteña.

Most people visit the Puna via the Tren a las
Nubes which climbs the Quebrada del Toro to San Antonio de los Cobres.
Our group traveled by the Sumaj Kay "vista cruiser" allowing us the opportunity
to stop along the way and photograph the spectacular scenery.

The sparse vegetation of the Quebrada del Toro
is a dramatic transition from the savanna and rain forest environments
seen in eastern and northern Salta Province. Salta and Jujuy provinces
are truly a land of transitions. Numerous geological and ecological systems
are found within a relatively small area. Most are readily accessible by
vehicle.

A typical vista on the Puna near the Salar del
Niño Muerto near San Antonio de los Cobres. The Puna/Altiplano is the world's
second largest continental plateau, after the Tibetan Plateau. Mean Puna
basin elevations exceed 3400 m. This is the only part of Argentina
where the indigenous population still constitutes a majority.

Numerous Neogene and Quaternary volcanoes dot
the Puna. Llama herds are abundant and vicuñas and chinchillas are
also common.

The Río Calchaquí cuts through
folded Neogene strata near Angastaco in southwestern Salta Province. The
Puna, Cordillera Oriental, and northern Sierras Pampeanas all converge
in this region along the western margin of the Cretaceous Salta Rift. Rocks
of the middle-late Miocene Payogastilla Group comprise the exposures in
the middle distance.

The middle Miocene Palo Pintado Formation, near
Angastaco, was a dynamic fluvial environment that recorded the uplift and
volcanic history of the western part of the Transition Zone.

Uplift of the Sierra de Quilmes (background)
deformed the strata of the Neogene Payogastilla Group in the Valles Calchaquíes
near Angastaco. The Sierra de Quilmes is the northernmost range of the
Sierras Pampeanas. Chronostratigraphic data suggest that it began to rise
in Pliocene time.

Strongly deformed, low-grade metamorphic rocks
of the Eocene Lumbrera Formation record the Neogene Andean uplift along
the Río Escoipe in the Cordillera Oriental. The road climbs to the
Cuesta del Obispo which has an elevation over 11,000'.

The road across the Cuesta del Obispo provides
magnificent views of the Cordillera Oriental. This photo was taken near
the Valle Encantado overlook.

A massive landslide threatens several ranches
and the road along the Río Escoipe in the Cordillera Oriental. The
slide, seen from the Cuesta del Obispo, will probably be activitated during
the next major earthquake. Most of this region has been seismically quiescent
during the last two centuries. Tremors were much more frequent when the
area was first colonized by the Spanish.

Neogene strata are spectacularly exposed in the
Quebrada de las Conchas between Salta and Cafayate. This vista point is
known as "El Balcon" because of its balcony-like overview of the river
canyon. The river drains the east side of the Puna and western Cordillera
Oriental, as well as the northern Sierras Pampeanas.

Strata of the Cretaceous-Tertiary Salta Group
are exposed in the Quebrada de las Conchas between Alemanía and
Cafayate. The strata were deposited in the Salta Rift Basin. The light-colored
Yacoraite Limestone is the source rock and reservoir rock for the hydrocarbon
resources found in the rift basin.

Strata of the Upper Miocene Guanaco Formation
crop out in the Río Yacones just north of the city of Salta. The
Cordillera Oriental rises in the background. The strata exposed in the
walls of the quebrada were deposited between 9-6 Ma.

Foreland basin strata from the Neogene Orán
Group are deposited on top of the Cretaceous-Paleogene Salta Group. This
photo shows the Upper Miocene Guanaco Formation exposed along the Río
Piedras near the boundary between the Cordillera Oriental and the Sistema
de Santa Bárbara in southern Salta Province.

The Río Metán drains the Sierra
de Metán, the easternmost range of the Cordillera Oriental in the
Transition Zone. Strata of the Salta and Orán Groups are exposed
in the riverbanks.

The Río Iruya exits the Cordillera Oriental and flows through the low, jungle-covered ranges of the Sierras Subandinas. In 1897, a new sugar mill near the town of San Ramón de la Nueva Orán was nearly flooded by the river's rising water. The owners sent engineers upstream to a place near Isla de Cañas, where the river was just a couple of hundred meters from a tributary of the Río Pescado. They constructed a diversion channel to send excess Río Iruya water into the Río Pescado. In 1898, the plan was put to the test. It worked but with an unexpected result. The 110 m difference in elevation between the two drainage systems caused the Río Pescado to capture the Río Iruya! This has resulted in approximately 100 m of downcutting since 1898, providing spectacular exposure Miocene through Pleistocene foreland basin strata in the canyon walls. The strata are more than 7 km thick! The nick point of the river has now migrated more than 15 km upstream (red area on map). The sugar mill has never flooded and is still in operation.